1. Field of the Invention
The present invention relates to a zoom lens adapted for use in a single lens reflex camera of so-called 35 mm Leica format, an electronic still camera, a video camera or the like, and more particularly to an ultra wide angle zoom lens of an F-number of about 2.8 with the image angle at the wide angle end of 90.degree. or larger.
2. Related Background Art
As an ultra wide angle zoom lens with the image angle exceeding 90.degree. at the wide angle end there are already known a 2-group zoom lens consisting of a front group of a negative refractive power and a rear group of a positive refractive power as disclosed in U.S. Pat. No. 4,477,153, and a zoom lens in which the rear group of a positive refractive power is divided into three groups of positive, negative and positive refractive powers, thus forming four groups in total, as disclosed in U.S. Pat. No. 5,000,550.
However, the former zoom lens described in U.S. Pat. No. 4,477,153 has an F-number of only about 3.5 in the examples described therein, and, if the aperture ratio is made larger, the lenses in the rear group become thicker, whereby the enough rear focal length becomes difficult to secure. On the other hand, the latter zoom lens described in U.S. Pat. No. 5,000,550 can attain a larger aperture ratio while securing a certain rear focal length, by dividing the rear lens group into three groups respectively of positive, negative and positive refractive powers and, in the zooming operation, maintaining the amount of movement of the negative lens group in the rear group less than that of other positive lens group, thereby reducing the amount of movement of the entire rear group.
However, in such zoom lens, as well as in the former one, the front lens group of negative refractive power tends to become huge in comparison with other lens groups because of the large incident angle.